I don't think we should delete an answer if the only problem is that it doesn't meet the hard science requirements.
In such cases they should be allowed to flounder in the usual low vote realms of the additional but not good enough answers that many questions get. Perhaps we should be a bit more liberal with the down votes, but a narrative answer in such cases may also be useful particularly if the asker isn't aware that any "correct" answer would require something like finite element analysis of turbulent flow.
The biggest problem with these answers is that they get more relative attention being on a hard-science question than the 30%-50% of answers on a "viral" question that get few or no votes.
We get very few hard-science questions and worrying about them and the small number of answers they get feels unnecessary. We want to keep the quality high, but I think the voting and flagging system is sufficient without addingputting additional manual load onto the moderators.
The hard-science flag should certainly remain in place unless the tag is removed.